Experimental study on the biodegradability of petroleum wastewater and improving the performance of a moving-bed biofilm reactor

This study explores applying a photocatalytic process as a pretreatment step before the attached-growth biological process in a Moving-Bed Biofilm Reactor (MBBR) using TiO₂ nanoparticles immobilized on Leca. The response surface method was used to optimize the photocatalytic process, focusing on parameters such as irradiation time (20–60 min), pH (4–7), photocatalyst concentration (30–50 g/L), and initial COD concentration of the effluent (400–1000 mg/L). The influences of these input parameters on the effluent biodegradability factor (BOD/COD) were assessed, showing the following impact percentages: irradiation time (23.34%), pH (22.95%), initial COD concentration (6.40%), and photocatalyst concentration (3.27%). After photocatalytic treatment, the biodegradability index of petroleum wastewater increased from 0.28–0.30 to 0.37–0.44, improving the overall treatment efficiency and reducing the required HRT in the subsequent biological process. Comparative experiments were conducted using the combined photocatalytic pretreatment and MBBR for two petroleum wastewater samples with COD concentrations of 400 and 1000 mg/L. Results indicated that neither the suspended activated sludge (AS) process nor MBBR alone could sufficiently reduce the organic load to acceptable levels. Experiments were performed with varying concentrations of activated sludge (MLSS: 3000, 4500, and 6000 mg/L) and hydraulic retention times (HRT: 6, 8, 10, and 12 h) at ambient temperature. Optimal conditions were determined at an MLSS concentration of 4500 mg/L and HRT of 12 h. Kinetic analysis based on first-order, second-order (Grau), and modified Stover-Kincannon models revealed compatibility of the moving-bed reactor with the Grau second-order and modified Kincannon models. The TiO₂-MBBR process demonstrated higher first-order and Grau second-order rate constants and exhibited a greater maximum specific removal rate (U_max) compared to other conventional processes in the literature such as AS, fixed-bed biofilm reactors, and upflow anaerobic fixed-film reactors.